📄 chemistry

Iodine Removal from CsI by FeCl2-Assisted Air Oxidation

This study demonstrates that adding FeCl₂ to CsI under air oxidation at 500°C for 4 hours achieves a 98.37% iodine removal efficiency by converting CsI to CsCl and volatilizing iodine as I₂, offering a promising method for treating spent nuclear fuel.

Original authors: Baoshan Ma, Yanhong Jia, Hui He

Published 2026-09-23

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CC BY 4.0

Original authors: Baoshan Ma, Yanhong Jia, Hui He

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Nuclear power plants generate electricity by splitting atoms, but this process leaves behind spent fuel that contains a mix of radioactive elements. Among these, iodine is a particular concern because one of its forms, iodine-129, remains radioactive for an incredibly long time and can easily move through the environment. If released, it tends to accumulate in the human thyroid gland, posing a significant health risk. To manage this, scientists must find ways to capture and remove iodine from the fuel before it becomes a hazard. In the high-heat environments where fuel is processed, iodine often binds tightly with cesium, another common byproduct, to form a stable solid compound called cesium iodide. This compound is difficult to break apart; simply heating it or blowing air over it is usually not enough to release the iodine, which remains trapped inside the solid structure.

Researchers at the China Institute of Atomic Energy set out to find a more effective way to break this bond and remove the iodine. They used cesium iodide as a safe, non-radioactive stand-in for the actual spent fuel to test different methods. Their initial experiments involved heating the material in the presence of strong oxidizing gases, such as pure oxygen or a mixture containing ozone. The goal was to see if these gases could force the iodine to turn into a gas and escape. However, the results were disappointing. Even with these powerful gases, the solid material barely changed, and very little iodine was removed. The stable structure of the cesium iodide held firm, proving that a stronger oxidizing atmosphere alone was not the key to unlocking the iodine.

The team then introduced a new element to the mix: a simple chemical called ferrous chloride. When they added this substance to the cesium iodide and heated the mixture in ordinary air, the outcome changed dramatically. The iodine, which had previously been stubbornly trapped, began to leave the solid and enter the air. The researchers found that the type of gas used mattered very little once the ferrous chloride was present; whether they used pure oxygen, ozone, or just regular air, the iodine removal efficiency was nearly the same. This suggested that the ferrous chloride was doing the heavy lifting, acting as a catalyst that made the reaction possible without needing expensive or complex gas supplies.

To find the perfect conditions for this process, the scientists varied the amount of time the mixture was heated, the temperature, and the ratio of the added chemical to the iodine compound. They discovered that the most effective setup involved heating the mixture to 500 degrees Celsius for four hours, using an equal amount of ferrous chloride and cesium iodide. Under these specific conditions, the method successfully removed nearly 98.4 percent of the iodine. The solid material left behind after the reaction was no longer the original cesium iodide. Instead, analysis showed it had transformed into two new substances: cesium chloride, which is a stable salt, and iron oxide, a reddish powder.

The study also revealed what happened to the iodine itself. Because the remaining solid contained no significant amount of iodine compounds, and because the chemical conditions favored the release of gas, the researchers concluded that the iodine had escaped primarily as elemental iodine gas. The process essentially swapped the iodine in the original compound for chlorine from the added chemical, creating a new solid salt while releasing the iodine into the air where it could be captured. This finding is significant because it demonstrates that a simple, low-cost chemical combined with ordinary air can achieve what powerful oxidizing gases could not on their own. Furthermore, the iron oxide left behind is weakly magnetic, which could potentially allow for easy separation and recovery of the iron from the final waste product. The work provides a practical, experimentally supported path for cleaning up iodine from nuclear fuel, relying on a straightforward chemical reaction rather than complex atmospheric controls.

Technical Summary: Iodine Removal from CsI by FeCl2-Assisted Air Oxidation

Problem Statement
Managing iodine isotopes, particularly the long-lived $^{129}\text{I}$ , is a critical challenge in spent nuclear fuel reprocessing due to its high mobility and radiological risk. In oxide spent fuel, cesium and iodine form stable cesium iodide (CsI), which resists volatilization under standard thermal conditions. Previous research indicates that simply increasing temperature or using strong oxidizing atmospheres (such as oxygen or ozone) is insufficient to effectively break the stable CsI lattice and release iodine. While advanced oxidation methods using $\text{NO}_2$ or gas-phase chlorination ( $\text{Cl}_2$ ) have shown promise, they present limitations such as the need for multiple reagent replenishments, surface moisture sensitivity, or equipment corrosion and off-gas purification burdens associated with chlorine gas. There is a need for a method that effectively converts stable CsI into volatile iodine species using accessible reagents and atmospheres without introducing significant secondary waste streams.

Methodology
This study utilized CsI as a non-radioactive surrogate for spent fuel to investigate oxidative volatilization. The experimental approach involved a thermogravimetric analyzer where CsI was treated under various conditions:

Atmosphere Comparison: Initial tests compared the efficacy of single oxidizing atmospheres (7% ozone and pure $\text{O}_2$ ) against air, both with and without the addition of ferrous chloride ( $\text{FeCl}_2$ ) as an auxiliary reagent.
Parameter Optimization: In the $\text{FeCl}_2$ -assisted system using air as the oxidant, the study systematically varied reaction time, the $\text{FeCl}_2/\text{CsI}$ molar ratio, and reaction temperature to determine optimal conditions.
Characterization: Residual iodine was quantified via ion chromatography to calculate removal efficiency. Solid products were analyzed using X-ray Diffraction (XRD) for phase identification, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) for morphology and elemental mapping, and X-ray Photoelectron Spectroscopy (XPS) to determine surface chemical states.

Key Results

Inefficacy of Single Oxidants: Direct oxidation of CsI using 7% ozone or pure $\text{O}_2$ at 300–400°C resulted in negligible weight loss. XRD analysis confirmed that the solid remained predominantly CsI, with only trace amounts of cesium iodate ( $\text{CsIO}_3$ ) formed. Iodine removal was ineffective.
Role of $\text{FeCl}_2$ : The addition of $\text{FeCl}_2$ significantly enhanced iodine removal. Under air, 7% ozone, and pure $\text{O}_2$ at 450°C, the average iodine removal efficiencies were 97.125%, 95.305%, and 96.880%, respectively. The similarity in efficiency across atmospheres suggests that the oxygen in air is sufficient for the reaction, and $\text{FeCl}_2$ is the primary driver of the conversion.
Optimal Conditions: The study identified the optimal parameters for the $\text{FeCl}_2$ $FeCl_{2}$ -assisted air oxidation system as:
- Molar Ratio ( $\text{FeCl}_2/\text{CsI}$ ): 1:1
- Temperature: 500°C
- Time: 4 hours
  Under these conditions, the iodine removal efficiency reached 98.37%.
Product Analysis: The solid residue was primarily composed of $\alpha\text{-Fe}_2\text{O}_3$ and $\text{CsCl}$ . XRD and XPS confirmed the oxidation of $\text{Fe(II)}$ to $\text{Fe(III)}$ and the conversion of $\text{CsI}$ to $\text{CsCl}$ . No significant crystalline phases of iodine-containing solids (e.g., $\text{CsIO}_3$ , $\text{Fe(IO}_3)_3$ ) were detected.
Reaction Mechanism: Based on the absence of hydrogen sources (using anhydrous reagents) and the volatility data, the study concludes that iodine was volatilized primarily as elemental iodine ( $\text{I}_2$ ). The proposed idealized reaction is:
$4\text{FeCl}_2 + 8\text{CsI} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + 8\text{CsCl} + 4\text{I}_2\uparrow$
The $\text{FeCl}_2$ serves a dual role: providing $\text{Cl}^-$ to convert $\text{CsI}$ to $\text{CsCl}$ and acting as a precursor for iron oxide formation upon oxidation by air.

Significance and Claims
The paper claims that this method provides a viable experimental pathway for high-temperature oxidative iodine removal from spent fuel surrogates. The primary significance lies in demonstrating that:

Air is Sufficient: High-purity oxygen or ozone generation is not strictly necessary; air serves as an effective oxidizing atmosphere when coupled with $\text{FeCl}_2$ .
Efficiency: The process achieves high iodine removal (>98%) without the equipment corrosion risks associated with gaseous chlorine or the complexity of $\text{NO}_2$ cycling.
Product Utility: The resulting solid residue contains $\alpha\text{-Fe}_2\text{O}_3$ , which is weakly magnetic, potentially allowing for the magnetic separation of the iron-containing residue from the cesium chloride product.
Mechanistic Insight: The study clarifies that the conversion of stable CsI requires a solid-phase reaction component ( $\text{FeCl}_2$ ) rather than just gas-phase oxidation, offering a new perspective on voloxidation strategies.

The authors present these findings as experimental support for developing iodine removal technologies, noting that the method retains cesium in the solid phase as $\text{CsCl}$ while effectively volatilizing iodine.

Technical Summary: Iodine Removal from CsI by FeCl2-Assisted Air Oxidation

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